Osseointegrated prostheses for rehabilitation of amputees (OPRA system) have been developed and commercialized by Integrum AB, Mölndal, Sweden. The OPRA system is currently used to anchor limb prostheses by means of a titanium implant that is surgically inserted into the bone. Generally, the OPRA system consists of a titanium screw implanted inside the bone and which serves as a fixture for the abutment, an abutment which is partly inserted inside the fixture and partly exiting the bone and stump so that the attachment of the artificial limb can be arranged at the head of the abutment, and an abutment screw for attaching the abutment to the fixture thus providing the mechanical coupling between the fixture and the abutment.
The connection device is arranged at the top of the prosthesis and forms the attachment between the abutment and the prosthesis. Specifically, it allows the attachment of the prosthetic component distal to the abutment. Also, the connector has a built-in safety mechanism to prevent bone damage in the event of excessive loads such as in a fall or other overload. All amputees are likely to have overloads or fall occasionally and a complication following osseointegration is the risk of bending the abutment. If the abutment is bent or deformed following a fall, then it must be replaced. And if the anchoring of the fixture is disturbed then there is a risk of loosening the fixture and a there has to be a new surgical operation.
In the event of unfavourable rotational loads being applied to the implant system it is previously known (see OPRA ROTASAFE system, also developed and commercialized by Integrum AB, Mölndal, Sweden) to design the built-in safety mechanism with two separate torque plates that are arranged to rotate relative to each another when the rotational load exceeds a release level. The amount of torque required to activate the connector device can be adjusted and graded to the status of the patient. The torsion release level of the connector device should be set to protect the implant system from significant rotational overload but still allow everyday activities without frequent release episodes that will disturb the patient and could increase wearing of the device. For instance the release level of the connector device could be altered between 10 Nm and 30 Nm, for instance by means of a number of press screws.
However, there is a need to reduce the outer dimensions of the connector device and also makes it more easy to set the release level instead of using as much as 8 press screws for controlling the release level.
There is also a need to take care of excessive bending forces, because in the event of an accidental fall not only rotational overload but also such unfavourable bending forces might occur. Even if it has been proposed to design a connector device with a release function for such excessive bending torques, there has been no success so far.